pmap: Implement pmap_ept_transform() for NVMM

pmap: Implement pmap_ept_transform() for NVMM

The pmap_ept_transform() transforms an initialized pmap structure to be
EPT type for Intel VMX hypervisor (e.g., NVMM) use.  This implementation
is derived from vmx_ept_init() and vmx_ept_pmap_pinit() in
'pc64/vmm/ept.c'.

Note that this function has a different prototype as NetBSD's one,
because we don't make 'pmap_ept_has_ad' a global variable so we need to
pass extra flags to the pmap.

When zeroing out the page directories, note that the valid area is two
pages if there is a pm_pmlpv_iso PTE installed (i.e., the system has
meltdown mitigation enabled), otherwise, it's only one page.  (credit to
Matt Dillon)